Evolution of the immune system

Thu Dec 3 13:33:08 EST 1992

Larry Moran comments on the following:


      Host defense mechanisms have been evolving since the first
      organisms encountered one another hundreds of millions of years
      ago. All multi-cellular organisms face the survival necessity of
      distinguishing between self and nonself in response to parasitism
      or infection with microbial pathogens. Studies of ancient defense
      systems have extensive implications for understanding the evolution
      of immunity and problems of human health and disease."

He says:
Yes, it would be wonderful to study ancient defense systems but unfortunately
they only exist in organisms that are now extinct (by definition). Until time
travel becomes feasible (don't hold your breath) we will have to content
ourselves with the study of modern organisms that are the product of three
billion years of evolution. I know that studies of defense mechanisms in many
organisms could lead to a better understanding of all of them, what I object
to is the implication that human defense mechanisms are advanced and modern
while others are ancient and primitive. We would all do well to remember that
evolution is best represented by a branching bush and not by a ladder with
vertebrates on the top.
   But Larry, is it not true that certain evolutionary developments
virtually arrest certain species in a frozen state so that other avenues
of evolution are closed off for them. Once the giraffe had decided to go
the route of elongating its neck it rapidly reached perfection regarding
its particular ecological nitch. Necks evolved long before the giraffe,
but if one wanted to study the properties of necks, the giraffe, rather
than Homo sapiens would seem an ideal example.

Larry goes on:
The defense mechanisms employed by bacteriophage and viruses are very
sophisticated and have been studied intensely. However, it is unlikely that
they are evolutionarily related to the immune system in mammals.
    I presume here you are talking about interactions of viruses with
their hosts. These are largely intracellular (e.g. the evolution of
restriction enzymes by bacteria and anti-restriction enzymes by phage).
Now, are you saying that the immune systems of mammals do not have an
intracellular component? What is all this business about intracellular
processing of protein antigens and peptide presentation in association
with MHC class I? Can we learn anything from the great facility of
bacteria to form inclusion bodies of expressed FOREIGN proteins? I think
you go too far in saying it is "unlikely" that this is evolutionarily
unrelated to mammalian intracellular immune mechanisms.
Larry goes on:                                            Similarly,
the mechanisms  employed by plants, arthropods and other invertebrates have
evolved over a period of hundreds millions of years from the time of the last
common ancestor of these organisms and vertebrates. Given that there is such
variety within vertebrates and that the immune systems are so sloppy it
doesn't seem very likely that there is much of an evolutionary history to
detect. Very few of the molecules of the vertebrate immune system have an
evolutionary history that predates the origin of chordates, this suggests
that most of the vertebrate immune system is fairly new on the scene.
    Well, Larry, what about the heat-shock proteins. Highly conserved and
guess where some of them map on mammalian chromosomes...the MHC complex!
                   Sincerely, Don Forsdyke

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